Do you read David Ramsey's book's? Or Suzy Orman? I really believe in what they teach...but maybe break the bonus down:
5%-Charity 10%-Your college fund 30%-Kids College Funds 30%-Pay Down Debt 15%-put in savings 10%-Blow money
One financial mistake we made with bonuses....we just put it all on debt. The problem with that is the next time you have an unexpected expense...there is no savings to use so you end up in debt again. But if you haven't....I suggest reading one of the above authors. I don't agree that getting out of debt makes it easier for her to leave. I believe it will actually have the total opposite affect.
First-By either saving the money and paying off debt you are acting financially responsible....that would be a large 180 for you.
Second-females (more so than males) have noted that they feel more secure when in a household with savings. I would venture a bet that your wife does not feel financially secure with you...which is probably adding to the fantasy with her supervisor.
I would really think about this one....in the end how you handle this may have a large impact on the future no matter which way you go.